Background: Breastfeeding peer support is advocated in national and international guidelines, but the evidence base is mixed. In the UK, breastfeeding peer support was found to be ineffective in randomised controlled trials, while women report positive impacts on breastfeeding experiences in qualitative studies. A key criticism levied against breastfeeding peer support is the lack of theory underpinning intervention design. Here we use the Behaviour Change Wheel to structure the analysis of evaluation data from an infant feeding peer support service in one area in North West England. We aimed to provide theoretically informed insights into how peer support can be operationalised to influence women’s breastfeeding experiences.

Methods: A 2 year mixed-methods evaluation (2014–2016) comprised surveys and interviews (individual or group) with peer supporters, health and community professionals, project leads and women, and routinely collected infant feeding data. We used the three layers (policies, intervention functions and behaviour-related components) of the Behaviour Change Wheel to structure and interpret the data.

Results: Overall data comprised 23 interviews (n = 14 - individual; n = 9 - group) and 409 completed surveys. The findings are presented in three sections. First, the ‘policies’ (outer) layer of the Behaviour Change Wheel provides insights into the existing context, infrastructure and resources that underpinned peer support delivery. Then the second (intervention functions) and inner (behaviour components) layers of the Behaviour Change Wheel are used to present three themes, ‘developing capabilities for infant feeding’, ‘motivating guidance and support’ and ‘opportunities for support’. These findings highlight that a peer support service delivered in a context of effective interdisciplinary partnerships, Baby Friendly Initiative accreditation, and flexible service planning, with peer support provided via different types of instrumental, social, practical and emotional support was perceived to be highly beneficial on women’s breastfeeding experiences. In the final section key challenges faced by the service are outlined.

Conclusion: While gaps and areas for development were highlighted, the service enhanced women’s capabilities, motivations and opportunities for breastfeeding. These theoretically informed insights into an organic and responsive peer support service help build the evidence base for breastfeeding peer support and to identify positive delivery features for future testing.